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GL1 I Matter KI1 b The crust GL1 I KI 1b crust • Specifications: • The crust is a thin layer of distinctive composition overlying the mantle; • Continental and oceanic crust can be recognised • And distinguished by differing thicknesses, composition and structure The crust OCEANIC and CONTINENTAL The crust • Solid rock • Very thin compared to size of Earth • If the Earth was the size of a football, the crust would be the equivalent thickness of a postage stamp stuck on it! • Density between 2.7g/cm3 to 3.2g/cm3 WARNING! • In OLD textbooks (and many recent geography textbooks!) • They refer to SIAL and SIMA • Please IGNORE these….. • They are based on outdated information and will be marked wrong in geology exams • They often also show continental crust floating on oceanic crust, this is also incorrect. CONTINENTAL CRUST • Forms the continents, and extends a little beyond them (the continental shelf and continental slope) • Mainly GRANITIC rock type • Density 2.7g/cm3 • Formed over a long period of time • Thickness varies, up to 70km thick in mountain ranges • Mountain ranges have “roots” Thick crust in mountain ranges Crust which is not as high has less underneath it ISOSTASY • Is the balance between the crust and mantle • As material is eroded off the mountain top, the root moves up as mantle flows underneath it. OCEANIC CRUST • Much thinner than continental crust • Average 6km • Younger than continental crust, max age 220 million years • Mainly composed of BASALT • Which is a slightly denser rock 3.0g/cm3 Age of ocean crust At the bottom of the crust • Boundary between the Crust and the Mantle is the • MOHO DISCONTINUITY • Named after Mohorovičić • Depth varies, deeper under mountains http://geology.com/articles/mohorovicicdiscontinuity.shtml • Moho shows a “jump” in the speed of P waves • It’s like them taking the motorway Discontinuities • How do you know that you have passed from the crust into the mantle? • Density change 2.7 or 3.0 to 3.3. • This change shows up as a change in seismic wave velocities and is called a discontinuity. Discontinuities • The waves are also refracted. • In particular this is called the Mohorovicic discontinuity or Moho. • Several other discontinuities exist within the Earth (but we are only expected to know the Moho) The crust is broken up into PLATES Where plates meet there is seismic activity Boundaries between plates: Two plates move together DESTRUCTIVE or CONVERGENT BOUNDARY Two plates move apart CONSTRUCTIVE or DIVERGENT BOUNDARY Controlled by CONVECTION CURRENTS in the Mantle • We’re leading into one of the great geological concepts • PLATE TECTONICS • (plates and the study of their movements) • GL1 II Energy KI 3